1. Field of the Invention
The present invention relates to a nitride semiconductor structure and a semiconductor light emitting device including the same, especially to a nitride semiconductor structure in which a stress control layer made from AlxInyGa1-x-yN is disposed between a light emitting layer and a p-type carrier blocking layer to improve crystal quality degradation caused by lattice mismatch between the p-type carrier blocking layer and the light emitting layer, increase the yield rate, and further reduce effects of compressive stress on quantum well layers. Thus electrons and holes are effectively confined in each quantum well layer and internal quantum efficiency is increased. Therefore the semiconductor light emitting device has a better light emitting efficiency.
2. Description of Related Art
In recent years, light emitting diodes (LED) have become more important in our daily lives due to their broad applications. LED is going to replace most of lighting devices available now and becoming a solid lighting element for the next generation. It's a trend to develop high energy saving, high efficiency and high power LED. Nitride LED has become one of the most popular optoelectronic semiconductor materials due to the advantages of compact volume, mercury-free, high efficiency and long service life. The wavelength of III-nitride covers almost covers the wavelength range of visible light so that it is a LED material with great potential.
Generally, for manufacturing nitride LED, firstly a buffer layer is formed on a substrate. Then an n-type semiconductor layer, a light emitting layer and a p-type semiconductor layer are formed over the buffer layer in turn by epitaxial growth. Next use photolithography and etching processes to remove a part of the p-type semiconductor layer and a part of the light emitting layer until a part of the n-type semiconductor layer is exposed. Later an n-type electrode and a p-type electrode are respectively formed on the exposed n-type semiconductor layer and the p-type semiconductor layer. A light emitting diode device is produced at last. The light emitting layer is in a multiple quantum well (MQW) structure formed by quantum well layers and quantum barrier layers disposed alternately. The band gap of the quantum well layer is lower than that of the quantum barrier layer, so that electrons and holes are confined in each quantum well layer of the MQW structure. Thus electrons and holes are respectively injected from the n-type semiconductor layer and the p-type semiconductor layer to be recombined with each other in the quantum well layers and photons are emitted.
However, the light efficiency of the LED can be affected by a plurality of factors such as current crowding, dislocation, etc. In theory, the light efficiency of LED is determined by external quantum efficiency, internal quantum efficiency and light-extraction efficiency. The internal quantum efficiency depends on material properties and quality. As to the light-extraction efficiency, it is defined as the ratio of the amount of light generated in the device and the amount of light escaping the device and radiated to the air. The light-extraction efficiency depends on the loss occurred while the light escaping the device. One of the main factors for the above loss is that the semiconductor material on the surface of the device has high refraction coefficient, so that total reflection occurs on surface of the material and photons are unable to be emitted. Once the light-extraction efficiency is improved, the external quantum efficiency of the semiconductor light emitting device is also increased. Thus various techniques for improving the internal quantum efficiency and the light-extraction efficiency have been developed in recent years. For example, the techniques include using indium tin oxide (ITO) as a current spreading layer, using the flip-flop, using patterned-sapphire substrate (PSS), using the current block layer (CBL), etc. Among the techniques used to improve the internal quantum efficiency, a method is to dispose a p-type carrier blocking layer (p-AlGaN) with high band gap between a multiple quantum well (MQW) structure and a p-type semiconductor layer. Thus more carriers are confined in the quantum well layers to increase electron-hole recombination rate and further improve light emitting efficiency. Therefore the brightness of LED is increased.
The MQW structure is generally formed by InGaN quantum well layers and GaN quantum barrier layers. Although the carriers can be effectively confined in the quantum well layers by using p-AlGaN as the p-type carrier blocking layer, there is high lattice mismatch between the p-AlGaN p-type carrier blocking layer and the GaN quantum barrier layer. Thus the InGaN quantum well layers are seriously affected by the compressive stress due to the lattice mismatch. The compressive stress changes band gap of each quantum well layer so that electrons and holes in the quantum well layers are separated from each other and the light emitting efficiency of the LED is reduced. Moreover, the compressive stress also degrades the adjacent GaN quantum barrier layers and interface properties among the InGaN quantum well layers so that carriers are lost at the interface and the light emitting efficiency of the LED is also affected.
Thus there is a room for improvement and a need to provide a novel nitride semiconductor structure and a semiconductor light emitting device including the same that overcome the above shortcomings.